A centrosomal protein STARD9 promotes microtubule stability and regulates spindle microtubule dynamics.

Centrosomal proteins play important roles in the spindle assembly and the segregation of chromosomes in the eukaryotic cells. STARD9, a recently identified centrosomal protein, was reported to influence the spindle pole assembly. However, the role of STARD9 in maintaining the stability and organization of microtubules are not known. Here, we show that STARD9 regulates the assembly and dynamics of both interphase and mitotic microtubules. The knockdown of STARD9 in HeLa or HCT116 cells with siRNA or shRNA induced a strong depolymerization of the interphase microtubules. The over-expression of the motor domain of STARD9 stabilizes microtubules against cold and nocodazole suggesting that STARD9 stabilizes microtubules in HeLa cells. Using fluorescent recovery after photobleaching, we showed that the knockdown of STARD9 strongly reduced microtubule dynamics in the live spindles of HeLa cells. The reassembly of microtubules in the STARD9-depleted cells was strongly reduced as compared to the microtubules in the control cells implying the role of STARD9 in the nucleation of microtubules. Further, the depletion of STARD9 inhibited chromosome separation and the STARD9-depleted HeLa cells were blocked at mitosis. Interestingly, the frequency of multipolar spindle formation increased significantly in the STARD9-depleted HeLa cells in the presence of vinblastine and the STARD9-depleted cells showed much higher sensitivity towards vinblastine than the control cells indicating a new approach for cancer chemotherapy. The evidence suggests that STARD9 regulates the assembly and stability of both interphase and spindle microtubules and thereby, play important roles in the cell cycle progression.